Project Summary Natural Killer (NK) cells are immune cells that very rapidly detect and eradicate infected or aberrant target cells. They constantly scan the surface of these cells for changes that would indicate infection or alteration by the tumor. NK cell receptor activation is achieved by binding to target cell expressed ligands that either activate or inhibit the NK cell. Engagement of inhibitory ligands prevents NK cell activation, thus allowing the healthy cell to leave unharmed. However, upon infection, activating ligands are now expressed by the target cell, leading to the activation of the NK cell and direct killing of the infected cell. Since viruses or tumors can actively prevent the expression of activating ligands in an effort to evade NK cell detection and killing we propose to disrupt the binding to inhibitory receptors by altering the structure of the inhibitory ligands. This in turn removes the brakes of NK cell inhibition, leading to NK cell activation and killing. Our strategy is built on our recent discovery that certain peptides bind and alter the structure of one class of ligands, termed Major Histocompatibility (MHC) class I molecules that engage inhibitory Killer Immunoglobulin receptors (KIRs). By preventing the interaction between these two cell-surface expressed proteins, we propose to re-activate otherwise inhibited NK cells. In this proposal we will study the universality of this structural change across the entire family of MHC I molecules, and test the activation of NK cells using designed peptides in a cell-based in vitro assay.